The subject of the present invention is a piezoelectric ceramic material based on lead zirconate titanate, which is distinguished by an excellent thermal and temporal stability of the functional characteristics.
Materials with high piezoelectric activity along with high thermal and temporal stability (high Curie temperature, low temperature coefficients and low ageing rates) of the functional characteristics are required for piezoelectric ceramics to be used for sensors, in particular in automotive engineering (knocking sensors, rotation-rate sensors, reversing sensors).
Piezoelectric ceramics with large deformation defects and high Curie temperature are required for the furthering of development in the field of multilayer actuators.
Piezoelectric ceramics materials have long been made of compositions based on the solid solutions (mixed crystals) of lead zirconate (PbZrO3) and lead titanate (PbTiO3). A variety of modifications of the basic system is possible as a result of substitution and/or addition of metal ions in limited concentrations if the respective ions fulfil prerequisites with respect to valency, ionic radius and character of chemical bonding.
By substitution in the original sense of the word is to be understood the partial replacement of the Pb2+ or Zr4+ and Ti4+ ions with ions of the same valency and similar ionic radii, such as Ba2+, Sr2+, Mg2+ and Sn4+. Such substitutions effect, on the one hand, an increase in the piezoelectric activity, but on the other hand, however, can also impair the thermal stability of the piezoelectric state.
A modification of the basic composition by doping with ions with a valency which is different from that of the original ions leads to a further diversification of the dielectric and electromechanical properties.
Ions from the xe2x80x9csoftenerxe2x80x9d group La3+, Bi3+, Sb5+, Nb5+ act in the basic system as donors and produce piezoelectric ceramics which are distinguished by a high dielectric constant and a high electromechanical activity, but are also characterised by high dielectric and mechanical losses as well as a dependency of the specific characteristics on strong electrical fields and mechanical loads.
A stabilisation of the piezoelectric ceramics based on lead zirconate titanate results from the doping with ions from the xe2x80x9chardenerxe2x80x9d group K+, Fe3+, Al3+: these ions act as acceptors and, in interacting with the ions of the basic system, effect a reduction of the dielectric and mechanical losses, but lead to a reduction in the dielectric constant, piezoelectric activity and specific electrical resistance.
As a result of coupled substitution of ions from the group of the xe2x80x9csoftenersxe2x80x9d with ions from the group of the xe2x80x9chardenersxe2x80x9d, it is, on the other hand, possible clearly to increase the stability of piezoelectric ceramics based on lead zirconate titanate whilst maintaining the piezoelectric activity and the high dielectric constant.
In conclusion, attempts have been made to do justice to the increased demands of the practical application of piezoelectric ceramics by means of multi-component systems, in which lead-containing ion complexes (xe2x80x9ccomplex compoundsxe2x80x9d) with the general formulation PbBxe2x80x21xe2x88x92xcex1Bxe2x80x3xcex1O3, with Bxe2x80x2: 5-valent or 6-valent cations and Bxe2x80x2: 2-valent cations (xcex1=⅓ or xc2xd depending on the valency of the cation Bxe2x80x2), partially replace the ion complex Pb2+(Zr4+, Ti4+)O3.
As a result of substitution of one or more of the complex compounds, which are also known as so-called relaxor-ferroelectrics, single-phase multi-component systems (for example as ternary or quaternary solid solutions) with perovskite structure result.
The substitution by lead-free compounds with perovskite structure, such as BiFeO3, KNbO3, NaNbO3, Na0.5Bi0.5TiO3 also leads to improvement in the properties of lead zirconate titanate ceramics.
These piezoelectric ceramics belong to the large family of (ceramic) ferroelectrics. Lead-free compositions such as (K, Na)NbO3, (Sr1xe2x88x92xBax)Nb2O6 are also known as ceramic ferroelectrics.
In all, there exists, as a results of the widest variety of modifications of the basic system of the solid solutions of lead zirconate titanate, a large multiplicity of compositions with which it has been possible in many cases to realise a specification of the dielectric and electromechanical properties of piezoelectric materials for different transformer functions that is suitable for the respective use.
From 1 Eyraud, L., Eyraud, P., Mathieu, J. C., Claudel, B. xe2x80x9cEffect of Simultaneous Heterovalent Substitutions on Both Cationic Sites on the Electrical Conductivity and Ageing of PZT Type Ceramicsxe2x80x9d (Ferroelectrics 50(1983)103-1 10), 2 Eyraud, L., Eyraud, P., Claudel, B. xe2x80x9cInfluence of Simultaneous Heterovalent Substitutions in Both Cationic Sites on the Ferroelectric Properties of PZT Type Ceramicsxe2x80x9d (J. Solid State Chem. 53(1984)266-272), 3 Ohenassion, H., Gonnard, P., Troccaz, L., Eyraud, L., Eyraud, P. xe2x80x9cCharacterisation de la stabilitxc3xa9 d""un element pixc3xa9zoxc3xa9lectrique du type PZT sous compression uniaxiale rapidexe2x80x9d (Revue Phys. Appl. 18(1983)479-486), and 4 Eyraud, L., Eyraud, P., Bauer, F. xe2x80x9cCurrent Research in the Field of PZT Ceramics and Ferroelectricxe2x80x9d (Polymers Adv. Cer. Mat. 1(1986)3, 223-231) is known the series of compositions
Pb0.995Sr0.015Ba0.005K0.0025Na0.0025(ZrxTi1xe2x88x92xNb0.02)O3(1+0.02)
x=0.52-0.54
The solid compositions were prepared unconventionally by oxalate mixed precipitation in a wet-chemical process. The aim of these investigations was the preparation of compositions with very low electrical conductivity as a result of compensation of the valency of heterovalent substituents, and ultimately in the stabilisation of the functional properties with respect to comparatively great mechanical loads. The optimal concentration of the substituents was found by trial and error, and as the most stable composition with respect to the mechanical loading capacity emerged the formulation with the Zr4+ content x=0.53. The compositions are characterised in 3 and 4. The stability criterion for these compositions was the resistance with respect to mechanical load. Details about the Curie temperature, temperature coefficients and ageing rates were not given. The optimal sintering temperature was given as 1230xc2x0 C. The wet-chemical preparation used in these works in order to prepare the compounds can be converted to a commercial scale only at considerable expense.
The underlying object of the present invention has been to prepare modified piezoelectric ceramics based on lead zirconate titanate with high depolarisation strength in the case of high mechanical (impact) loads (for gas igniters) and in particular with low temperature coefficients and ageing rates of the functional characteristics, high Curie temperature (for sensors) and large deformation effect (for actuators).
In this connection, these piezoelectric ceramics are to be able to be synthesised by way of the conventional mixed-oxide route and sintered at temperatures below 1150xc2x0 C.
This object has been achieved by a piezoelectric ceramic material based on lead zirconate titanate having the characterising features of the first or second claim. Preferred developments are characterised in the subclaims.
Surprisingly, it has been established that, in the case of lead zirconate titanate (with perovskite structure A2+B4+O3), the desired stabilisation of the materials can be achieved, and simultaneously the reduction of the sintering temperature can be rendered possible, by partial substitution with ferroelectrically active compounds.
In accordance with the invention, in order to do this, alkaline earth metals, preferably Sr2+ and/or Ba2+, alkali metals K+, Na+, and metals Nb5+, Sb5+, Ta5+, known per se in modifications of lead zirconate titanate (perovskite structure A2+B4+O3), are built in stoichiometrically by way of ternary or quaternary solid solutions by way of lead-free compounds having the general composition
A1+B5+O3 or
A2+Bxe2x80x20.251+Bxe2x80x30.755+O3
for heterovalent ion combinations (with respect to A2+B4+O3) with per-ovskite structure, and possibly in combination with an alkaline earth niobate (Ba1xe2x88x92xSrx)2Nb2O7 (pyrochlore type). If appropriate, in order to increase the piezoelectric activity of the compositions in accordance with the invention, it is also possible to work with an excess of xe2x80x9csoftenerxe2x80x9d ions (non stoichiometry less than 1% by weight).
The general formulation of the ternary or quaternary solid solutions formed in accordance with the invention in this way is:
{(1xe2x88x92u)Pb(ZrxTi1xe2x88x92x)O3.uA2+(Bxe2x80x20.251+Bxe2x80x30.755+)O3}+wMe25+O5xe2x80x83xe2x80x83Type 1
(1uxe2x88x92v)Pb(ZrxTi1xe2x88x92x)O3xe2x88x92uA1+B5+O3xe2x88x92v(SR1xe2x88x92yBay)2Nb2O7xe2x80x83xe2x80x83Type 2
In addition, a combination of alkaline earth metal ions, or the Sr2+ and Ba2+ ions in a certain concentration ratio, preferably the combinations Sr0.7Ba0.3, Sr0.75Ba0.25, or Sr0.8Ba0.2, particularly the combination Sr0.75Ba0.25, can stand for the cation A2+ in Type 1, and the cation A1+ in Type 2 can be represented by K+, Na+ in a certain concentration ratio, preferably by the combinations K0.4Na0.6, K0.45Na0.55, K0.5Na0.55, K0.55Na0.45, K0.6Na0.4, particularly preferably by the combination K0.5Na0.5.
The ceramic materials of Type 1 in accordance with the invention can be obtained by modification of lead zirconate titanate with complex, lead-free perovskites having the general composition A2+Bxe2x80x20.251+Bxe2x80x30.75 5xe2x88x92O3 (compound of alkaline earth metal ions, preferably Ba2+ and/or Sr2+ in combinations of alkali ions Bxe2x80x2=alkali metal such as K+ or Na+ with 5-valent metal ions Bxe2x80x35+=Nb, Ta, Sb), wherein
40xe2x89xa6xxe2x89xa60.55
0 less than uxe2x89xa60.10, and
0xe2x89xa6wxe2x89xa61% by weight
Me5+=Nb5+, Ta5+, Sb5+.
Type 2 in accordance with the invention can be obtained by modification of lead zirconate titanate with double oxides (perovskite) having the general composition A1+B5+O3 of alkali metals, preferably of K+ and/or Na+, with pentavalent metals, for example with the metal ions Nb5+, Ta5+, Sb5+, preferably with Nb5+, in combination with an alkaline earth niobate of the pyrochlore type, preferably with a (Ba1xe2x88x92xSrx)2Nb2O7, wherein
0.40xe2x89xa6xxe2x89xa60.55
0 less than u,vxe2x89xa60.05, and
0xe2x89xa6yxe2x89xa61.0.
Surprisingly, it has been established that the ceramic materials in accordance with the invention are distinguished by an excellent thermal and temporal stability of the functional characteristics. In this connection, the ceramic materials in accordance with the invention are distinguished by the following stability criteria:
The ternary and quaternary solid solutions with heterovalent substituents (ion combinations with charge compensation) in accordance with the invention can be prepared as a stoichiometric, single-phase composition by way of the conventional route of the so-called mixed oxide technique in total synthesis.
Surprisingly, the ceramics prepared in this way sinter at temperatures below 1150xc2x0 C., so that evaporation of PbO is substantially counteracted.
Materials with low sintering temperature, large expansion effect and high Curie temperature that are suitable even for multilayer actuators are available in certain composition ranges with the materials in accordance with the invention.
The materials in accordance with the invention are distinguished in particular by low ageing rates and temperature coefficients of the functional characteristics and are therefore particularly suitable for sensors.
On account of their high depolarisation strength with repeated mechanical impact loading, the materials in accordance with the invention are also suitable for ignition elements.
The stabilised piezoelectric ceramic materials in accordance with the invention with high piezoelectric activity can preferably be used for the sensor technology and actuator technology and in certain cases for ignition elements.